1. Field of the Invention
The present invention is directed to a valve timing control for an internal combustion engine, and more particularly to an improved valve timing control for an internal combustion engine that includes a variable valve timing mechanism.
2. Description of Related Art
A marine drive such as an outboard motor includes a marine propulsion device powered by an engine. The propulsion device typically is a propeller and is submerged when an associated watercraft rests on a body of water. The outboard motor can employ either a two-cycle engine or a four-cycle engine. Recently, however, many outboard motors are offered with four-cycle engines because they provide better emissions control.
Typically, a four-cycle engine includes one or more intake and exhaust valves moving between an open position and a closed position. One or more camshafts can be provided to actuate the valves in a timed manner. When the intake valves are open, air is introduced into combustion chambers of the engine through the intake ports. When the exhaust valves are open, exhaust gases are discharged from the combustion chambers through the exhaust ports.
The engine typically includes one or more throttle valves to regulate an amount of air delivered to the combustion chambers. The throttle valves can be operated by the operator with a throttle lever. In general, the more the throttle valves open, the more power is output from the engine.
The engine can include a hydraulically operated variable valve timing (VVT) mechanism that can change the opening and closing timing of the respective valves by changing an angular position of the camshaft or camshafts. Typically, the VVT mechanism has a plurality of hydraulic chambers and vanes movably disposed within the hydraulic chambers. The vanes define two spaces together with the chambers and a working fluid such as, for example, oil is delivered to and discharged from the respective spaces. The vanes thus move between inner walls of the chambers which regulate a fully retarded angular position and a fully advanced angular position. That is, the inner walls define mechanical stoppers of the vanes. With the vanes moving in the chambers, the angular positions of the camshaft is changed between the fully retarded angular position and the fully advanced angular position.
A control device such as, for example, an electronic control unit (ECU) is used to control the VVT mechanism under various control strategies. For instance, the ECU controls the VVT mechanism either to set the valve timing at the fully advanced position for relatively high engine speeds to ensure high charging efficiency and high performance of the engine, or to set the valve timing at the fully retarded position for relatively low engine speeds to ensure high combustion efficiency, fuel economy and good emission control. Otherwise, the ECU controls the VVT mechanism to set the valve timing at a position between the fully advanced position and the fully retarded position in response to a running condition of the engine.